fix(animations): do not leak DOM nodes/styling for host triggered animations (#18853)

Closes #18606

PR Close #18853

packages/core/test/animation/animation_integration_spec.ts

@@ -733,6 +733,46 @@ export function main() {
              flushMicrotasks();
              expect(fixture.debugElement.nativeElement.children.length).toBe(0);
            }));
+
+        it('should properly evaluate pre/auto-style values when components are inserted/removed which contain host animations',
+           fakeAsync(() => {
+             @Component({
+               selector: 'parent-cmp',
+               template: `
+                <child-cmp *ngFor="let item of items"></child-cmp>
+              `
+             })
+             class ParentCmp {
+               items: any[] = [1, 2, 3, 4, 5];
+             }
+
+             @Component({
+               selector: 'child-cmp',
+               template: '... child ...',
+               animations:
+                   [trigger('host', [transition(':leave', [animate(1000, style({opacity: 0}))])])]
+             })
+             class ChildCmp {
+               @HostBinding('@host') public hostAnimation = 'a';
+             }
+
+             TestBed.configureTestingModule({declarations: [ParentCmp, ChildCmp]});
+
+             const engine = TestBed.get(ɵAnimationEngine);
+             const fixture = TestBed.createComponent(ParentCmp);
+             const cmp = fixture.componentInstance;
+             const element = fixture.nativeElement;
+             fixture.detectChanges();
+
+             cmp.items = [0, 2, 4, 6];  // 1,3,5 get removed
+             fixture.detectChanges();
+
+             const items = element.querySelectorAll('child-cmp');
+             for (let i = 0; i < items.length; i++) {
+               const item = items[i];
+               expect(item.style['display']).toBeFalsy();
+             }
+           }));
       });
 
       it('should cancel and merge in mid-animation styles into the follow-up animation, but only for animation keyframes that start right away',